Biofeedback sensor



FIG. 1 is a front perspective view of a biofeedback sensor, showing my new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a top plan view thereof; and,

FIG. 4 is a rear elevational view thereof.

The left and right side views of the system, as well as the bottom view of the system, form no part of the claimed design. 

I claim the ornamental design for a biofeedback sensor, as shown. 